1. Field of the Invention
The present invention relates to an antenna apparatus for carrying out data communication in noncontact between the antenna apparatus and a data carrier used by being attached to a person or goods, for example, for control of entrance and evacuation or automatic sorting of goods. More particularly, the present invention relates to an antenna apparatus increasing a receiving capacity in noncontact communication.
2. Description of the Related Art
Normally, a noncontact RF tag (hereinafter, referred to as RF tag) used as a data carrier comprises a coil antenna and an IC chip including an involatile memory. Data communication is carried out by wireless between the RF tag and an antenna apparatus which generates an induced magnetic field. The technology of noncontact communication attracts attention as a recognition technology substituting for a bar code. In the RF tag enabling the noncontact communication, a power source for operating the IC chip is taken out from the induced magnetic field generated by the antenna and therefore, a battery is dispensed with.
Meanwhile, a loop antenna is used for the antenna apparatus and various antenna shapes are constituted depending on methods and location of use thereof.
For example, there is present an antenna apparatus arranged with one antenna provided to be opposite to one side of a detecting region, or a gate type antenna apparatus arranged with two antennas having the same shape provided on both sides of a detecting region so that they are opposite to each other. The antenna apparatus are used for an automatic reading apparatus of goods carried by a conveyer, a reading apparatus for inventory control for entrance and evacuation of goods, or a related-art security gate of preventing shoplifting.
An antenna apparatus used for such an object is requested to read the RF tag within the detecting region without being dependent on a position or a direction thereof and needs to realize the reading within a limited radio wave output in compliance with the radio law. In order to satisfy the conflicting requests, a number of modes of antennas have been proposed in the related-art antenna apparatus using radio wave.
When an investigation is given on a communicating function of an antenna of this kind, in the case that transmission and reception can be carried out by one antenna, normally, the antenna is set to adapt to a transmitting frequency and therefore, a receiving sensitivity is lower than that in an optimum case. Therefore, there is a case of providing a receiving antenna set to adapt to a receiving frequency separately from a transmitting antenna or a transmitting and receiving antenna.
For example, as shown in FIG. 6, when 1 loop of a receiving antenna 62 formed in a rectangular shape is arranged on an inner side of 1 loop of a transmitting antenna 61 similarly formed in a rectangular tape, a current 63 is induced to couple at the receiving antenna 62 by a magnetic field generated in a direction of penetrating a loop shape of the transmitting antenna 61. Therefore, a current flowing in the transmitting antenna 61 is reduced by an amount of being consumed by being coupled to the receiving antenna 62.
As a countermeasure against the reduction in the current, as shown by FIG. 7, a transmitting antenna 71 having a first loop antenna 71a and a second loop antenna 71b in a 8-like shape is provided. When a current having a phase inverse to a phase of a current flowing in the second loop antenna 71b is made to flow in the first loop antenna 71a, a current 73a induced in a receiving antenna 72 by a magnetic field generated by the first loop antenna 71a and a current 73b induced in the receiving antenna 72 by a magnetic field generated by the second loop antenna 71b are provided with the same magnitude and inverse directions of currents. Therefore, the current 73a and the current 73b are canceled by each other. Thereby, coupling of the transmitting antenna 71 and the receiving antenna 72 is eliminated to thereby prevent the reduction in the current of the transmitting antenna 71.
The above-described shape of 2 loops is not limited thereto but as shown by FIG. 8, there can also be provided a transmitting antenna 81 of a 8-like shape which is formed in a rectangular shape, an upper side of which is provided for a first loop antenna 81a and a lower side of which is provided for a second loop antenna 81b. The first and second loop antennas 81a, 81b are formed by narrowing a central portion of the transmitting antenna 81 to an interval by which the 2 loops are not intersected with each other.
Further, when considering a case of 3 loops of a transmitting antenna, as shown by FIG. 9, a transmitting antenna 91 has a first through a third loop antenna 91a through 91c arranged in series in a rectangular shape. In the first loop antenna 91a and the third loop antenna 91c at both end portions of the transmitting antenna 91, currents having a phase inverse to a phase of a current flowing in the second loop antenna 91b is made to flow.
In this case, when a receiving antenna 92 in the rectangular shape is arranged on a plane the same as that of a transmitting antenna 91 and on an inner side of the transmitting antenna 91, respective magnetic fluxes generated by the transmitting antenna 91 are made to pass a first through a third magnetic flux passing region S1, S2, S3 on the inner side of the receiving antenna 92 in correspondence with the first through the third loop antennas 91a through 91c. At this occasion, a direction of magnetic fluxes in the first and the third magnetic flux passing regions S1, S3 is inverse to a direction of magnetic fluxes of the second magnetic flux passing region S2.
When the magnetic fluxes of the respective magnetic flux regions S1, S2, S3 are respectively designated by notations φ1, φ2, φ3, a total φ of the fluxes passing the first through the third magnetic flux passing regions S1, S2, S3 of the receiving antenna 92 becomes φ1−φ2+φ3.
Normally, a relationship of a degree of canceling when the magnetic fluxes passing the magnetic flux passing regions of the receiving antenna 92 are canceled by each other is not φ1+φ3=φ2. Therefore, the total φ of the magnetic fluxes is not nullified. Therefore, a current is induced in the receiving antenna 92, a current flowing in the transmitting antenna 91 is consumed to reduce by the receiving antenna 92 and thus a transmitting function is reduced. When such an inappropriate coupling cannot completely be canceled, a communicating function is reduced and a region of detecting the RF tag is narrowed.
Further, JP-A-2002-237720 discloses a technology of achieving excellent communication by using 4 loops of a transmitting antenna and minimizing a nondetecting region produced at a portion of intersecting loop antennas. However, when a receiving antenna can be a related-art rectangular shape is provided to increase a receiving function of the antenna apparatus, there is a case in which a transmitting function is reduced owing to the above-described fact that the total of the magnetic fluxes is not nullified.